I am in need of help on my 1989 ish Mercury Force 120. I have ignition issues that I can not seem to properly diagnose. I will go back and give an explanation as to how I got to this point… the point of wanting to smash this thing into many little pieces…
The engine ran great for many years and then being a Force, it lost compression due to piston and ring failure. I did a complete engine rebuild, new pistons, new crank, all new bearings, all new seals and gaskets. After the rebuild, the engine ran for me but it had a dead misfire on cyl#2 and an erratic misfire on cyl#4. After going through the CDI diagnostic sheets and schematics I found that swapping the switchboxes around had no effect on the cyl’s misfire and when I tested the stator and trigger, they both tested normal… I figured it was a faulty trigger. I replaced the trigger with a used part and the misfires continued, I then changed the switchboxes with used parts. The dead miss on #2 was corrected and the erratic miss on #4 now became erratic on #3. So, I know that used parts can be defective and I know that my knowledge of this engine is limited…. I just need it to run
I’ve also done further testing on the trigger for this engine using a timing light. With plugs out and grounded and cranking to set timing I used the timing light to show the trigger point of the cyls. I marked the flywheel for each cyl and while cranking, placed the timing light pickup on #1 and it flashed on my #1 mark, #2 flashed only on #2, #3 flashed all 4 cylinders, #4 flashed only on #4. So I thought that can’t be right and changed the trigger back to my original trigger and re-tested. I got the same results. Not sure why… Maybe I’m missing something or maybe there’s more to it…
I took the boat out onto the water to test as I thought maybe I just need to run it for a while and clear out some of the old fuel. On the water the engine ran rough, misfiring and not responding to throttle as it should have. It did run good though at WOT.
I should also mention that this is a Prestolite system.
I pulled the spark plugs to try and read them and I also thought maybe there was moisture on them but wasn’t sure. It looked like tiny water droplets but otherwise the new plugs where dark brown and black on the face… not washed looking.
The method I use to determine if I have a misfire is to use my timing light and place the pickup on the plug wire while running or cranking and if the light flashes its good and if it doesn’t flash it’s a misfire. So, my understanding is if it was a fuel related misfire it would still spark and read on my timing light as a spark.
So, my questions are
1, Am I testing the trigger properly and do I still have a faulty trigger?
2, Is there an ignition conversion to a newer style system that’s more reasonably priced for parts?
3, Is there something I’m missing in my diagnoses?
4, Are there alternative switchboxes to use… cheaper ones
5, What else can I test?
Any help would be greatly appreciated!
The engine ran great for many years and then being a Force, it lost compression due to piston and ring failure. I did a complete engine rebuild, new pistons, new crank, all new bearings, all new seals and gaskets. After the rebuild, the engine ran for me but it had a dead misfire on cyl#2 and an erratic misfire on cyl#4. After going through the CDI diagnostic sheets and schematics I found that swapping the switchboxes around had no effect on the cyl’s misfire and when I tested the stator and trigger, they both tested normal… I figured it was a faulty trigger. I replaced the trigger with a used part and the misfires continued, I then changed the switchboxes with used parts. The dead miss on #2 was corrected and the erratic miss on #4 now became erratic on #3. So, I know that used parts can be defective and I know that my knowledge of this engine is limited…. I just need it to run
I’ve also done further testing on the trigger for this engine using a timing light. With plugs out and grounded and cranking to set timing I used the timing light to show the trigger point of the cyls. I marked the flywheel for each cyl and while cranking, placed the timing light pickup on #1 and it flashed on my #1 mark, #2 flashed only on #2, #3 flashed all 4 cylinders, #4 flashed only on #4. So I thought that can’t be right and changed the trigger back to my original trigger and re-tested. I got the same results. Not sure why… Maybe I’m missing something or maybe there’s more to it…
I took the boat out onto the water to test as I thought maybe I just need to run it for a while and clear out some of the old fuel. On the water the engine ran rough, misfiring and not responding to throttle as it should have. It did run good though at WOT.
I should also mention that this is a Prestolite system.
I pulled the spark plugs to try and read them and I also thought maybe there was moisture on them but wasn’t sure. It looked like tiny water droplets but otherwise the new plugs where dark brown and black on the face… not washed looking.
The method I use to determine if I have a misfire is to use my timing light and place the pickup on the plug wire while running or cranking and if the light flashes its good and if it doesn’t flash it’s a misfire. So, my understanding is if it was a fuel related misfire it would still spark and read on my timing light as a spark.
So, my questions are
1, Am I testing the trigger properly and do I still have a faulty trigger?
2, Is there an ignition conversion to a newer style system that’s more reasonably priced for parts?
3, Is there something I’m missing in my diagnoses?
4, Are there alternative switchboxes to use… cheaper ones
5, What else can I test?
Any help would be greatly appreciated!
